1. Field of the Invention
This invention relates to an improvement of a semiconductor device comprising double or multiple layers of passivation films and more particularly to a technique of passivating a semiconductor device with silicon compounds.
2. Description of the Prior Art
Generally, in a unit circuit element like a transistor, a diode, a semiconductor resistor, a capacitor, etc. or in a so-called integrated semiconductor device like an integrated circuit device composed by assembling many such circuit elements as described above, interconnecting the same and providing outgoing lead terminals thereto, the surfaces thereof and the parts which particularly affect the characteristics thereof, e.g., the PN junction parts, the part nearby which becomes a space charge layer, the region operating due to the diffusion of minority carriers, etc. are covered with a passivation film like a SiO.sub.2 film because the characteristics of said elements or devices are worsened by the influence of external moisture, conducting materials, ionic materials or the like.
For a passivation film to be formed directly on a semiconductor surface, SiO.sub.2 has been mainly used, but recently nitrides like Si.sub.3 N.sub.4 are developed as a substitute for SiO.sub.2. Generally, in SiO.sub.2, the surface part of a Si substrate under the film has somewhat a tendency to turn to N conductivity type and said tendency is greater in Si.sub.3 N.sub.4. According to the experiment of the present inventors, the increment of the surface electron density in the former is 3 .times. 10.sup.11 electrons/cm.sup.2, whereas in the latter case, the increment is 3 .times. 10.sup.12 electrons/cm.sup.2 and the ratio is 1 : 10. Accordingly, a method wherein Si.sub.3 N.sub.4 is formed directly on a semiconductor surface is not always preferable. Further, though said SiO.sub.2 is suitable for forming the film directly on a semiconductor surface on account of the small surface electron density, it has the disadvantage that the variation of the characteristics is frequently caused because metallic ion, for example, sodium ion can easily move in the SiO.sub.2 film. Said defect is caused by the following mechanism. When ionic materials like Na.sup.+ enter SiO.sub.2, the materials move in SiO.sub.2 at a relatively low temperature (e.g. above 100.degree. C.) and the movement thereof is remarkably forced by the application of an electric field. Thus, the characteristics are changed during the maintenance operation at a high temperature. Therefore, it is necessary to prevent the entrance of harmful ions like Na.sup.+ as much as possible in forming said SiO.sub.2 passivation film. On the other hand, though the direct formation of a Si.sub.3 N.sub.4 film on a Si substrate surface has the disadvantage described hereinabove, Si.sub.3 N.sub.4 has the advantage that the movement of ionic materials like Na.sup.+ inside Si.sub.3 N.sub.4 is quite small.
Another semiconductor device comprising double layers of passivation films consisting of a silicon oxide film and a silicon nitride film has been proposed. In such a semiconductor device, however, if a hole is provided in the double film consisting of a silicon oxide film and a silicon nitride film when fabricating a semiconductor device, excessive etching or side etching of the lower layer, where the etching speed is faster, i.e., the silicon oxide film, presents due to a difference in etching rate between the two, and thereby the fabrication of the device becomes very difficult.